24.3.4 Technical questions
1. Why is the fluidized bed operated at a higher temperature ?
Ans: The advantage of fluidized bed reactor is to achieve maximum mass transfer and if this principle is better exploited then it is also possible to alter the selectivity of the process. Therefore, fludiized bed is operated at 600oC so that no maleic anhydride is produced and a higher yield of pthalic anhydride is obtained.
2. Is it justified to use spray of the feed in the case of fluidized bed reactor where as packed bed reactor received the vaporized feed stream ?
Ans: Well, since fludization involves better mass and heat transfer effects when compared to packed bed reactor and since fludized bed reactor is operating at a far higher temperature than the packed bed reactor, it is easy for us to understand that the sprayed feed (i.e., naphthalene) will vaporize instantaneously and will undergo the desired reaction. Also, it is worthy to note that for better facilitating these variations, the gas hour space velocity and air to feed ratio of the fluidized reactor is chosen totally different from those existing in the packed bed reactor.
3. How do switch condensers assist in generating the crude pthalic anhydride solid product ?
Ans: The switch condensers operate like an on and off switch involving the crystallization of pthalic anhydride and once these crystals are formed they are allowed for partially melting the same (by offing the condensation process) and this way, the flakes melt from the condenser tubes to obtain as a crude pthalic anhydride.
When the crystallization is on, it implies that cooling fluid is circulated and when crystallization is off, then hot fluid is circulated. These are circulated in an on and off mode and hence we got the name as switch condensers.
4. Why are the switch condensers not called as switch crystallizers but switch condensers ?
Ans: Here, the vapors are received above the dew point of the pthalic anhydride and they are subjected to cooling. Since phthalic anhydride is sublime (solid generating vapour), the crystallization process is called as switch condensation.
5. In the flowsheet shown, the non-condensible gases such as CO2 are not shown to be vented out. Where do you think they will be vented out and why ?
Ans: CO2 is vented out along with non-condensible gases from the switch condensers. Other than this, there is no other place to remove the same as the product gases are sent to these units only. In case of packed bed reactor technology, the vent also consists of maleic anhydride and fumaric acid which are absorbed in a scrubber using water as a solvent.
References:
1. Dryden C. E., Outlines of Chemical Technology, East-West Press, 2008
2. Kirk R. E., Othmer D. F., Encyclopedia of Chemical Technology, John Wiley and Sons, 1999-2012